Design Article
Discrete audio amplifier basics - Part 1: Bipolar junction transistor circuits
John Linsley Hood
3/10/2010 2:00 PM EST
FETs, usually referred to as MOSFETs (metal oxide silicon FETs), because of their method of construction. These devices are available in both P type—operating from a negative supply line—and N type—operating from a positive supply line.
BJTs and MOSFETs are also available in small-signal and larger power versions, whereas FETs and MOSFETs are manufactured in both enhancement-mode and depletion-mode forms. Predictably, this allows the contemporary circuit designer very considerable scope for circuit innovation, by comparison with electronic engineers of the past, for whom there was only a very limited range of vacuum tube devices.
In addition, there is a very wide range of integrated circuits (ICs), which are complete functional modules in some (usually quite small) individual packages. These are designed both for general-purpose use, such as operational amplifiers, and for more specific applications, such as voltage regulator devices, current mirrors, current sources, phase-sensitive rectifiers, and an enormous variety of designs for digital applications, which mostly lie outside the scope of this book.
In the case of discrete devices, I think it is unnecessary for the purposes of audio amplifier design to understand the physical mechanisms by which the devices work, provided that their would-be user has a reasonable grasp of their operating characteristics and limitations and, above all, a knowledge of just what is available.
Next: Junction Transistors
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abraxalito
3/10/2010 9:05 PM EST
I thought that Doug Self as editor would have caught the myth being propagated here that the BJT is a 'current operated device'. He makes plain in his own writings that its not, rather its voltage operated with an exponential law between VBE and IC.
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d-jeff
3/11/2010 3:32 AM EST
The bipolar IS voltage controlled with an exponential law (even though when a VBE is applied, some IB takes place). In this article on bipolar, this fact does not seem obvious, I agree with previous comment.
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bcarso
3/11/2010 12:17 PM EST
Hood was a delightful character, but wasn't always entirely clear on solid-state physics. I recall at one point in his description of JFET operation he refers to majority carriers "tunneling" through the pinched-off channel...
Although there are situations where a voltage-controlled analysis can be better suited to a given device, Barrie Gilbert makes the point that really both voltage and current are always involved (see his way-best-of-book articles in Toumazou's (et al., eds) collection Analogue IC Design: the current-mode approach). Another very powerful and seldom-referenced approach, analysis in terms of charge as the variable, is presented in Ed Cherry's Amplifying Devices and Low-pass Amplifier Design, which does an integrated presentation of tubes and transistors.
An aside: Barrie G. also mentioned once some experimental heterojunction ADI bipolars that had a beta of order 100k.
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Guru of Grounding
3/11/2010 7:32 PM EST
Arguing about whether a bipolar transistor is voltage or current "operated" is like arguing about how many angels can dance on the head of a pin! Notice that the context of his statement included the word "linearity". Many transistors have very linear hFE over many decades of current while Ic vs Vbe is anything but linear. Linearity is THE prime consideration in audio after all. Circuit designers simply want to characterize a transistor as a 3-terminal device and, for linear applications, the quantum physics involved inside are of little interest. I have disagreements with Mr. Hood, but they're about other issues. I think his explanation is quite appropriate in its context.
Bill Whitlock
president/chief engineer
Jensen Transformers
www.jensen-transformers.com
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bcarso
3/16/2010 4:06 PM EDT
Agreed Bill---although I would suggest that the expression should be "very constant beta" rather than "linear" beta, as a function of collector current. "Linear" beta for example would be where beta was given by an expression like m*Ic + b.
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c_abraham
6/12/2010 7:07 PM EDT
Then why does every semiconductor OEM call the bjt a current controlled device? Do you know more then them? How many semicond physics courses have you taken? Nothing personal, but if what you say is true, every OEM would concur, & every university semicond course would teach the same.
At the "black box" viewpoint, a bjt is classified as current controlled. For a deeper look involving internal charge distribution, depletion regions, etc., the charge control model is used, for the bjt & FET as well. Ultimately, at the atomic level, only quantum mechanics can describe what is happening. This is the universal model. Shockley stated this in the early 1950's.
Again, at the external viewpoint, current control is a good 1st order approximation. More in depth study produced the charge control model. Ultimately the device is best described by QM. Every OEM cannot be wrong. I took 1 undergrad course in semicond phy from the phy dept. Then I took 4 couses in semicond phyfrom the EE dept. in my MS & Ph.D. studies at 2 different universities. The bjt & FET are described as I just stated.
A bjt cannot be directly driven from a low impedance voltage source. Thermal runaway would take place. We always "current drive" a bjt. If the input signal source is constant voltage type, then a resistor must be used to drive the bjt. This is why bjt is "current driven". The Vbe is of course, absolutely needed, as are Ib & Ie. All 3 are needed for transisto action to occur. Without Ib, Vbe, & Ie, there can be no Ic. They are equally important.
Claude
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pcsalex
7/15/2011 9:47 AM EDT
by the way on figure 7B that is not a rearranged cascaded, but a differential amplifier!
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